1. Field of the Invention
The present invention relates to a CVD reactor and to a process for producing an epitaxially coated semiconductor wafer.
2. The Prior Art
Crystal growth from vapor is employed in semiconductor technology, in particular, for producing epitaxially coated semiconductor wafers. The term epitaxy describes the growth of a monocrystalline layer on the planar boundary surface of a monocrystalline substrate, generally a substrate wafer, for example a semiconductor wafer.
This coating or deposition is carried out using so-called chemical vapor deposition (CVD) in CVD reactors, as for example described in EP 0,714,998 A2. The semiconductor wafer is in this case firstly heated using heat sources and then exposed to a gas mixture, referred to below as the process gas, consisting of a source gas, a carrier gas and, where appropriate, a dopant gas. Trichlorosilane is for example used as the source gas, and hydrogen, for example, is used as the carrier gas. The dopant gases for doping the epitaxial layer are gaseous compounds, for example from Group III or VI of the periodic table. These compounds, for example phosphine or diborane decompose, as the source gas does, in proximity to the heated wafer. The foreign atoms are then included or incorporated in the crystal lattice of the epitaxial layer. As a rule, the semiconductor wafer (substrate wafer) and the epitaxial layer are doped differently in order to obtain a sharp change in the electrical properties, for example a steep rise in the resistance profile on transition from the substrate wafer to the epitaxial layer.
When producing an epitaxial layer, it is necessary to suppress so-called autodoping. The term autodoping describes the undesired contamination of the process gas with the substrate dopant (dopant of the semiconductor wafer). At elevated temperature, the substrate dopant is capable, because of thermal motion, of migrating in the host crystal lattice. This migratory movement is also referred to as diffusion. When an epitaxial layer is being deposited on the front of the semiconductor wafer, the substrate dopant can diffuse out through the back of the semiconductor wafer. This gaseous compound mixes with the process gas and becomes deposited in a CVD process, so that the dopant of the semiconductor wafer finally ends up in the epitaxial layer.
At this point the front and back of a semiconductor wafer ought to be defined. Before an epitaxial layer is deposited on one side of a semiconductor wafer, the wafer undergoes chemical and/or mechanical surface treatments, for example lapping, polishing and etching treatments. The front of the semiconductor wafer, on which at least one epitaxial layer is deposited, is given a very shiny polish, while the back is coated, for example, with polycrystalline silicon and/or silicon dioxide. The polysilicon layers are used, for example, as extrinsic getters, and the oxide layers, for example, as a protective layer.
According to the prior art, various processes have been disclosed which counteract autodoping. By depositing, for example, an oxide or monocrystalline or polycrystalline or amorphous protective layer on the back of the semiconductor wafer, substrate dopant can be prevented from diffusing out during the epitaxy.
As a rule, use is made of an oxide layer which is deposited in an LTO process (low-temperature oxide process) upstream of the epitaxy. Because of their relatively low process temperatures, LTO processes are time-consuming and expensive. After this back coating, however, the LTO (low-temperature oxide) is also found on the wafer edges and, to some extent, on the front of the wafer as well, which impairs polishing and epitaxy processes. It is therefore necessary, in a subsequent process step, to remove the LTO from the edges and the front by wet chemical means, preferably by etching. However, the growth of a perfect epitaxial layer requires a planar boundary surface, so that the front needs to be polished after the wet chemical treatment.
It is known to the person skilled in the art that an oxide protective layer can also be obtained by exposing the semiconductor wafer to gases with oxidizing action, for example oxygen or ozone at elevated temperatures.
A disadvantage with all known processes for producing an epitaxially coated semiconductor wafer having a protective layer to suppress autodoping is the additional process steps which need to be carried out in different reactors, treatment baths and polishing lines.